Active Noise Control (ANC) systems attenuate undesired noise using feedforward and feedback structures to adaptively remove undesired noise within a listening environment, such as within a vehicle cabin. ANC systems generally cancel or reduce unwanted noise by generating cancellation sound waves to destructively interfere with the unwanted audible noise. Destructive interference results when noise and “anti-noise,” which is largely identical in magnitude but opposite in phase to the noise, combine to reduce the sound pressure level (SPL) at a location. In a vehicle cabin listening environment, potential sources of undesired noise come from the engine, the interaction between the vehicle's tires and a road surface on which the vehicle is traveling, and/or sound radiated by the vibration of other parts of the vehicle. Therefore, unwanted noise varies with the speed, road conditions, and operating states of the vehicle.
A Road Noise Cancellation (RNC) system is a specific ANC system implemented on a vehicle in order to minimize undesirable road noise inside the vehicle cabin. RNC systems use vibration sensors to sense road induced vibrations generated from the tire and road interface that leads to unwanted audible road noise. This unwanted road noise inside the cabin is then cancelled, or reduced in level, by using speakers to generate sound waves that are ideally opposite in phase and identical in magnitude to the noise to be reduced at the typical location of one or more listeners' ears. Cancelling such road noise results in a more pleasurable ride for vehicle passengers, and it enables vehicle manufacturers to use lightweight materials, thereby decreasing energy consumption and reducing emissions.
RNC systems are typically Least Mean Square (LMS) adaptive feed-forward systems that continuously adapt W-filters based on both acceleration inputs from the vibration sensors located in various positions around a vehicle's suspension system, subframe and body, and on signals of microphones located in various positions inside the vehicle's cabin. Certain driving events, such as driving over train tracks, hitting a pothole, and driving over a speedbump, induce signals in both the accelerometers and the microphones. Consequently, the LMS RNC system will adapt the W-filters to attempt to more optimally cancel these signals, which have a different spectral character than that of the surrounding pavement. However, these types of events are transients, and are not indicative of most of the road that the vehicle is traveling on. Therefore, when the W-filters are adapted based on these transient, non-stationary events, the RNC is worsened for a period of time after the events. This is because the RNC system needs to re-adapt to re-converge to the correct W-filters to optimally cancel the steady-state or pseudo-steady state road surface.